The invention relates to transducers, and more particularly, to an improved dielectric coating for transduction drivers.
Acoustical transducers convert electrical energy to acoustical energy, and vice-versa, and can be employed in a number of applications. For example, transducers are a primary component used in sonar applications such as underwater seismic prospecting and detection of mobile vessels in, such applications, acoustic transducers are generally referred to as projectors and receivers. Projectors convert electrical energy into mechanical vibrations that imparts sonic energy into the water. Receivers are used to intercept reflected sonic energy and convert the associated mechanical vibrations into electrical signals. Multiple projectors and receivers can be employed to form arrays for detecting underwater objects.
A projector typically includes an electromechanical stack of ceramic or rare earth elements having a particular crystalline structure. Depending on the crystal structure and material, a projector may be, for example, piezoelectric, electrostrictive, or magnetostrictive. For instance, if a ceramic crystal is subjected to a high direct current voltage during the manufacturing process, the ceramic crystal becomes permanently polarized and operates as a piezoelectric. An electrical signal applied to the ceramic crystal generates mechanical vibrations. A plurality of such crystals can be configured in a stack to provide greater vibrations, and is commonly referred to as a xe2x80x9cdriverxe2x80x9d or xe2x80x9ctransduction driver.xe2x80x9d
In another instance, direct current voltage can be temporarily applied to a ceramic stack during operation to provide polarization of the crystals. Under such conditions, the operation of the projector is electrostrictive. After the application of direct current voltage is discontinued, the electrostrictive ceramic stack is no longer polarized, and vibrations stop. In a third instance, a magnetostrictive stack is exposed to a direct current magnetic field via a coil and the stack material magnetic domains are aligned. An electrical signal applied to the coil causes the stack to generate vibrations.
One type of projector is a flextensional sonar projector, which is typically a low frequency transducer. Low frequency acoustic signals are desirable because they are less attenuated by the water through which they travel, which allows the signals to travel great distances. A flextensional transducer includes a piezoelectric, electrostrictive, or magnetostrictive driver housed in a mechanical projector shell. Vibration of the driver is caused by application of an alternating electrical signal, which produces magnified vibrations in the shell thereby generating acoustic waves in the water. The shell vibrations are dependent upon the piezoelectric, electrostrictive, or magnetostrictive properties of the driver.
The driver is typically coated with a dielectric coating to prevent corrosion of the stack elements if the internal environment of the transducer becomes exposed, and to ensure that the high voltage actuating signal is delivered to the driver and not short-circuited. Such a short-circuit may be from lead to lead of the driver, or from one or both of the driver leads to ground.
A typical technique for coating a transduction driver consists of applying a single acrylic coating. This coating limits the drive voltage field that can be applied to the transducer, which is typically about 10 V/mil. Conventional acrylic coatings are associated with many other disadvantages as well. For example, acrylic is marginally water resistant, has less than optimal adhesion and a low thermal breakdown temperature, and has only moderate dielectric strength.
What is needed, therefore, is a transduction driver coating material that has a high water resistance, a high breakdown temperature, and high dielectric strength.
One embodiment of the present invention provides a coating for transduction drivers. The coating includes one or more coats of a polyester varnish that has been degassed. Each coat is applied to a transduction driver under vacuum. In one such embodiment, the one or more coats of degassed polyester varnish include a first coat of the degassed polyester varnish that is applied to the transduction driver under vacuum, and a second coat of the degassed polyester varnish that is applied to the transduction driver under vacuum while the first coat is still tacky. The one or more coats of degassed polyester varnish may further include a third coat of the degassed polyester varnish that is applied to the transduction driver under vacuum after the first and second coats are cured. In one particular embodiment, the polyester varnish has a dry dielectric strength of 1000 V/mil or more, and a viscosity under 200 CPS. The first, second, and third coats may be cured by air drying at room temperature.
Another embodiment of the present invention provides a method of coating a transduction driver. The method includes degassing a polyester varnish thereby forming a degassed polyester varnish. The method further includes applying a first coat of the degassed polyester varnish to the driver while under vacuum, and then applying a second coat of the degassed polyester varnish to the driver while the first coat is still tacky under vacuum. The method proceeds with curing the first and second coats of degassed polyester varnish. The method continues with applying a third coat of the degassed polyester varnish to the driver under vacuum after the first and second coats are cured, and curing the third coat of degassed polyester varnish. The polyester varnish can be, for example, Dolph""s AC-43. Note that the degassing and applying steps can be performed simultaneously, where the degassing is achieved by virtue that the polyester varnish is applied under a vacuum.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.